Plasma torch having cylindrical velocity reduction space between electrode end and nozzle orifice

ABSTRACT

A plasma torch, capable of cutting in a dross free state, is made possible by increased energy density of the arc jet. The operation efficiency is not reduced even with a low operating gas flow rate, since the arc jet can be stably maintained in the plasma torch. The torch has a high double arc resistance and excellent durability. This is realized by forming a velocity reduction space N from near a lower end (3b) of the electrode (3) to a nozzle (9) at the front end of the plasma torch (1), the velocity reduction space being used for reducing the axial velocity component of the operating gas which flows along the outer periphery of an electrode (3). The velocity reduction space (N) is cylindrically shaped, and the diameter (Dd) of the cylindrical shape is larger than the diameter (da) of a lower end (3b) of the electrode (3). The velocity reduction space can be formed such that the diameter (Dd) of the cylindrical shape is larger than the diameter (da) of the lower end (3b) of the electrode and larger than the height (Ha) of the cylindrical shape. The energy density of the arc jet is greater than 4×10 5  A·S/kg.

TECHNICAL FIELD

The present invention relates to a plasma torch, and, more particularly,to a plasma torch in which a transferred arc jet is produced to cut aworkpiece.

BACKGROUND ART

Hitherto, there has been a demand for a plasma torch which is capable ofcutting material, such as steel, stainless steel, etc., with highprecision and without adherence of molten metal. (hereinafter referredto as dross), which has a narrow cutting width, which is even capable ofcutting thick plates, and which has a long life. With regard to suchprior art, one of the present applicants has proposed a transferredplasma torch, for example, in Japanese Utility Model Application No.1-72919. For example, each of FIGS. 7 and 8 is a cross-sectional view ofa nozzle and electrode section of a conventionally proposed transferredplasma torch, wherein swirling air currents are produced in theoperating gas. In the transferred plasma torch 50 of FIG. 7, a switch 53is operated to transfer the arc, formed between a nozzle 52 and anelectrode member 51a of an electrode 51, to a workpiece 54 to be cut. Inthis plasma torch 50, a swirler member 55 is inserted near the electrode51, disposed within the nozzle 52, and a plurality of holes 55a areobliquely formed downwardly therein. The operating gas, which has passedthrough the plurality of holes 55a, becomes swirling currents and issuccessively accelerated in an acceleration section 52a, formed into a Vshape with a gentle inclination at the front end of the nozzle 52, andreaches a nozzle restriction section 52b for restricting the arc let 56such that it moves in a straight line.

In plasma torch 60 of FIG. 8, a swirler member 63 is inserted near anelectrode 62, disposed in nozzle 61, and a plurality of holes 63a areformed in the swirler member 63 perpendicular to axial center Z of theplasma torch 60 and tangential with respect to the inner peripheral faceof the swirler member 63. At the front end of the nozzle 61 below theelectrode 62, there is disposed a velocity reduction space 61a below andapart from the lower end of an electrode member 62a of the electrode 62.The operating gas, which has passed through the plurality of holes 63a,becomes swirling air currents; and in the velocity reduction space 61a,these swirling air currents allow arc jet 56 to be held in alow-pressure space formed in the center axis and therearound. Since thenozzle 61 has the velocity reduction space 61a at the upstream side, itis capable of preventing deflection of the arc jet 56 which is ejectedfrom the nozzle restriction section 61b, so that it is generated with ahigh degree of straightness, which results in excellent cutting of theworkpiece 54.

However, in such above-described conventional transferred plasmatorches, when in conventional use a current is made to flow through anelectrode and a conventional operating gas flow rate is supplied, it isextremely difficult to achieve cutting of a workpiece in a dross freestate. This is thought to be very difficult to achieve even when theconditions are changed.

Another different prior art is known, in which cutting in a dross freestate is achieved by a method which comprises cutting a workpiece by anarc jet having the operating oxygen gas further enveloped by an oxygencurtain during cutting (refer, for example, to Japanese Patent Laid-OpenNo. 59-229282). However, the use of oxygen for the curtain results inincreased gas consumption as well as a reduced precision in thedimensions of the cut face or the like due to burning.

The present invention has been achieved to overcome the above-describedproblems of the prior art, and relates to a plasma torch and, moreparticularly, to a plasma torch in which a transferred arc jet isgenerated, wherein dross adhesion does not occur, the arc jet is stable,and the nozzle, etc., has a long life.

DISCLOSURE OF THE INVENTION

Accordingly to a first aspect of the present invention, there isprovided a plasma torch having a velocity reduction space formed nearthe lower end of an electrode toward the nozzle at the front end of theplasma torch, the velocity reduction space being used for reducing theaxial velocity component of the operating gas flowing along the outerperiphery of the electrode. The velocity reduction space is cylindricalin shape, the cylindrical shape having a diameter greater than thediameter of the lower end of the electrode. The velocity reduction spacecan be formed such that the diameter of the cylindrical shape is largerthan the diameter of the lower end of the electrode, and, at the sametime, larger than its own height. Further, the operating gas, made intoswirling currents by a swirler member, is caused to flow through acylindrically-shaped annular entrance section, the entrance sectionbeing formed almost parallel to the outer periphery of the electrode,through a thin conically-shaped annular acceleration section, theacceleration section being formed at the tapered section of theelectrode, through the velocity reduction space, through a conicalacceleration Space, the conical acceleration space being formed belowthe velocity reduction space, and then through a restriction sectionwithin a cylindrical nozzle. The operation gas, formed into currents, isthen ejected toward the workpiece.

With a construction wherein the velocity reduction space is formed nearthe lower end of the electrode, it is possible to maintain most of thearc jet within the plasma torch in the velocity reduction space, whichresults in increased stability of the arc jet in the plasma torch. Inaddition, since the diameter of the velocity reduction space is largerthan the diameter of the lower end of the electrode, there is lessfluctuation of the arc jet in the radial direction in the plasma torch,that is, the arc jet becomes more stable with less wandering. This meansthat the thickness of the gas insulation layer is increased in theradial direction, making it possible to prevent the occurrence ofimproper discharges, such as double arcs. Further, since the diameter ofthe cylindrical shape is larger than its height, the length in the axialdirection of the arc jet, held in the velocity reduction space, becomesrelatively small, making it possible to prevent kink instability, etc.,when the arc jet is being extended. Still further, since the operatinggas flows through the entrance section, the acceleration section, thevelocity reduction space, the acceleration space, and the restrictionsection, it is possible to achieve smooth flow of the operating gas andto maintain the stability of the arc jet in the plasma torch at the sametime.

According to a second aspect of the invention, there is provided aplasma torch in which an operating gas flows therein and is formed intoswirling currents by a swirler member, the currents being caused to flowfrom the end of an electrode along the outer periphery of a taperedportion of the electrode toward a workpiece, and in which an arc isdeveloped by the electrode and ejected as an arc jet from a nozzle atthe front end of the plasma torch toward the workpiece. In thisconstruction, the energy density of the arc jet is greater than 4×10⁵[(ampere×second)/kg]. In this case, the energy density I/m of the arcjet is defined as I/m [arc current value I (ampere)/operating gas flowrate m (kg/s)], and m will hereinafter represent the flow rate of theoperating gas (in kg) per unit time (in seconds).

With such construction, steel and other materials can be cut by means ofan arc jet with a high energy density, thereby making it possible toperform cutting in a dross free state.

According to a third aspect of the invention, there is provided a plasmatorch having a swirler member with a plurality of ejection holes formedtherein on a plane substantially perpendicular to the central axis ofthe plasma torch, the swirler member causing the generation of jets withonly a swinging velocity component V.sub.θ in the tangential directionand the formation of operating gas into swirling currents. This plasmatorch has a substantially cylindrically-shaped velocity reduction space,and has the following dimensions: 0≦Hd≦7De, 30°≦φ≦100°, 90°≦θ≦150°,0.5De≦Ha≦2.5De, 4De≦Dd≦10De, -0.4De≦Hb≦0.6De, and 2.5De≦Hc≦4De. Here, Derepresents the nozzle orifice diameter.

With a construction wherein the plasma torch has a velocity reductionspace formed into a predetermined dimensional shape, it is possible toperform cutting in a dross free state, and, at the same time, a desireddesign can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a cross-sectional view of the front end of a nozzle of theplasma torch in accordance with the present invention;

FIG. 1b illustrates reference characters denoting the dimensions, etc.,of FIG. 1a;

FIG. 2 illustrates swirling currents of operating gas flowing from theswirler member of FIG. 1a;

FIG. 3 illustrates reference characters designating the dimensions,etc., of the nozzle front end of the conventional plasma torch of FIG.8;

FIG. 4 shows experimental results of the dross adhesion height whenchanges are made in the operating gas flow rate and the cuttingvelocity;

FIG. 5 illustrates experimental results of the number of double arccumulative occurrences;

FIG. 6 shows experimental results of the dross adhesion height whenvarious changes are made in the diameter of the nozzle in the presentinvention;

FIG. 7 is a cross-sectional view of the nozzle front end of aconventional plasma torch;

FIG. 8 is a cross-sectional view of the nozzle front end of anotherconventional plasma torch;

FIG. 9 shows experimental results of the relationship between parallelsection length/nozzle diameter and static pressure in the presentinvention;

FIG. 10 shows experimental results of the relationship between velocityreduction space height/nozzle diameter and static pressure in thepresent invention; and

FIG. 11 illustrates experimental results of the relationship between thenozzle diameter length/nozzle diameter and the double arc occurrencelimiting current in the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will be given of a preferred embodiment of the plasmatorch of the present invention with reference to the attached drawings.

FIG. 1a is a cross-sectional view of the nozzle front end of a plasmatorch, while FIG. 1b shows reference characters designating thedimensions, etc., of FIG. 1a. An electrode 3 is provided at the axialcenter of a plasma torch 1. An insulation member 5 is providedconcentrically to and outwardly of the electrode 3, and a swirler member7 and a nozzle 9 are provided outwardly of the insulation member andconcentrically to the electrode 3.

The electrode 3 is a conductive member of, for example, copper. Theelectrode member 3a, made of hafnium, tungsten, silver, or the like, isembedded in the substantially central part of the front end of theelectrode 3. The lower end 3b of the electrode 3 is a plane sectionhaving a diameter da, which is greater than the outer diameter of theelectrode member 3a. A tapered section E (taper angle α) extendsupwardly from the lower end of the electrode 3 toward an electrode outerdiameter db.

The insulation member 5 is made of an insulation material, such asceramic, and electrically insulates the electrode 3 from the nozzle 9.The inner peripheral face of the insulation member 5 is tightly fittedto a portion of the electrode 3 having the outer diameter db, and theouter peripheral face of the lower portion of the insulation member 5has a swirler member 7 of inner diameter Da fitted tightly thereto. Asupply gas passage 11 is formed between the outer periphery of theportion of the insulation member 5 having an outer diameter dc and theinner periphery of the portion of the nozzle 9 having an inner diameterDb. A gas passage 13 is formed from the swirler member 7 and below alower end 5a of the insulation member 5.

The swirler member 7 is formed of a material having excellenthigh-temperature resistance and processability, such as free-cuttingsteel and copper. The inner peripheral face is tightly fitted to theinsulation member 5, and the outer peripheral face is tightly fitted tothe inner peripheral face of the nozzle 9 which has an inner diameterDb. The outer periphery of the swirler member 7 has formed therein gaspath slits 7a at two or more places at equal distances apart along thecircumference. In addition, holes 7b, serving as ejection holes, areformed therein at equal distances apart, extending from the slits 7atoward the inner peripheral dimension, as shown in FIG. 2, and beingsubstantially tangential with respect to the annular supply gas path 13in a plane (the X-Y plane in FIG. 2) which is substantiallyperpendicular to the longitudinal axis. Although in this embodiment theouter periphery of the swirler member 7 is slightly cut to form a path,it is noted that the axial center of the holes 7b is not more than ±5°,and preferably not more than ±3° in the vertical dimension (verticaldimension in FIG. 1a). The holes 7b are formed below the lower end 5a ofthe insulation member 5.

The nozzle 9 is formed of conductive material such as an iron-containingmaterial, a copper-containing material, and a stainless steel. The innerperipheral face with the inner diameter Db has the outer peripheral faceof the swirler member 7 tightly fitted thereto, with one end face 7c ofthe swirler member 7 being in contact with the nozzle 9. The upperportion of the nozzle 9 is connected to a plate (not illustrated), andis removably secured with screws, etc., to the torch body (notillustrated). The inner face of the nozzle 9 having the diameter Dc,which is substantially equal to the inner diameter Da of the swirlermember 7, is nearly parallel to the face of the electrode 3 having theouter diameter db, and the length of the parallel section is Hd. Acylindrically-shaped annular space, formed by the inner face of thenozzle 9 having the diameter Dc and the outer peripheral face of theelectrode 3 having the diameter db, is called the entrance section L. Itis noted that the outer peripheral face of the electrode 3 at theentrance section L can have a tapered lower outer diameter section. Forexample, it can have a tapered section E.

The nozzle 9 has a tapered section M, tapering downwardly and inwardlyfrom the inner diameter Dc to the nozzle front end, which forms an angleφ, which can be either nearly equal to or greater than the taper angle αof the electrode 3. Even below this tapered section M and near theelectrode lower end 3b (distance in the axial center dimension), thereis formed a cylindrical section (hereinafter referred to as the velocityreduction space N). The velocity reduction space N is concentric withthe longitudinal axis of the electrode 3 and is cylindrical in shapewith a diameter Dd, which is greater than the diameter da of the lowerend 3b of the electrode 3, and with a height Ha, which is smaller thanthe diameter Dd. It is noted that, with regard to the distance Hb in thelongitudinal axial dimension between the upper end of the cylindricalshape of the velocity reduction space N and the electrode lower end face3b, while the lower end 3b of the electrode 3 is illustrated in FIG. 1bas being above the velocity reduction space N, the lower end 3b of theelectrode 3 can be positioned in the velocity reduction space N. In thiscase, the velocity reduction space N has its upper end formed as acylindrically annular shape.

A tapered section (hereinafter referred to as the acceleration space P)tapers downwardly and inwardly from the diameter Dd of the velocityreduction space N at an angle θ, and the tapered section merges into anozzle orifice formed at the end of the nozzle 7 and having a diameterDe. A predetermined size is selected for the nozzle orifice diameter Dein accordance with the material of the workpiece, the thickness of theworkpiece, the cutting width precision, etc. The length Hc of the nozzleorifice having the diameter De is also selected in the same way.Hereafter, the nozzle orifice 9a is defined by both the orifice diameterDe and the orifice length Hc.

With each of the components arranged in the above-described manner, theoperating gas takes the path summarized below. It flows from the annularentrance section L, having almost parallel cylindrical walls formed bythe outer periphery of the electrode 3 and the inner periphery of theswirler member 7 and the nozzle 9, and then downwardly through the thinconically annular acceleration section (hereinafter referred to as theacceleration section M), which has tapered inner and outer faces formedby the tapered section E of the electrode 3 and the tapered section M ofthe nozzle 9, and which is connected to the entrance section L at agentle angle. The operating gas then reaches the cylindrically shapedvelocity reduction space N, formed at the end of the accelerationsection M and near the lower end 3b of the electrode. After havingflowed into the velocity reduction space N, the operating gas passesdown through the acceleration space P, located below the velocityreduction space N, then through the nozzle restriction section 9a,formed as a cylindrical shape at the front end of the nozzle 9, and isejected to a workpiece (not illustrated) in the form of an arc jet.Although, in the above-described construction, examples of materials foreach of the component members were given, they are not to be construedas limitative.

A description will be given of the operation of the plasma torch ihaving the above-described construction. The operating gas flows fromthe supply gas path 11, formed between the outer diameter dc of theinsulation member 5 and the inner diameter Db of the nozzle 7, and thenthrough the slits 7a of the swirler member 7, through the holes 7b,formed in the swirler member 7 at equal distances apart, and through thegas path 13, located inwardly of the gas path 11. As shown in FIG. 2,the gas, flowing out from the plurality of equal holes 7b, flows as jetsin the form of tangential swirlers, having only a tangential velocitycomponent Vθ. The tangential swirlers, which pass from the gas path 13to the entrance section L, become uniform swirling currents of operatinggas, and flow downwardly into the acceleration section M, connected tothe entrance section L at a gentle angle. The swirling currents,accelerated in the acceleration section M, flow into the velocityreduction space N, formed near the lower end 3b of the electrode 3. Inthe velocity reduction space N, the arc jet (hereinafter referred to asthe arc column) is stably held with respect to the electrode axis, usingthe low pressure gradient of the swirling central portion symmetrical tothe axis, generated by the swirling current produced by the tangentialswirler; that is, the pressure gradient symmetrical to the axis producedby the centrifugal force of the current swirling velocity component(becomes minimum on the center axial line). Here, in the velocityreduction space N, as the path area increases, the axial velocitycomponent decreases, while the swirling velocity component, which doesnot decrease, remains at an appropriate value, so that it is possible tocreate the necessary steep pressure gradient symmetrical to the axis tostably maintain the arc column. Since the velocity reduction space N hasa large diameter Dd, the distance between the outer edge of the arccolumn (current boundary) and the velocity reduction space N wall islarge, which results in an increased gas insulation layer thickness, soas to increase resistance to double arc and thus restrict the generationof double arcs. This increases the durability of the plasma torch.

The operating gas is gradually accelerated within a short distance andnarrowed down from the velocity reduction space N to the nextacceleration space P, so that the arc column, maintained with respect tothe electrode axis in the velocity reduction space N, is narrowed downand flows into the nozzle restriction section 9a. In the nozzlerestriction section 9a, the operating gas becomes a predetermined arcjet and travels a short distance from the electrode 3 to the workpiece.Accordingly, a shorter distance from the lower end 3b of the electrode 3to the entrance of the nozzle restriction section 9a causes the arccolumn to be maintained at a shorter length, thus reducing theoccurrence of various instabilities of the arc column formed in thecurrent, such as arc column wandering.

A description will be given of experiments performed on the plasma torch1 in accordance with the present invention, described in detail above,and the conventional plasma torch 60 proposed by the present inventor.

EXPERIMENTAL EXAMPLE 1: Dross Adhesion Height

In this experiment, swirling currents were generated and theconventional plasma torch 60 having the velocity reduction space 61a(see FIG. 8) was used to examine the dross adhesion height when changeswere made in the operating gas flow rate and the cutting velocity. Thisexperiment was conducted to show that, in the case of the conventionalplasma torch with a nozzle and an electrode, it is difficult to increasethe energy density I/m of the arc jet since the double arc generationlimiting current is small; and it is particularly necessary to increasethe energy density I/m of the arc jet when cutting steel plates using aplasma torch utilizing transferred arc jets, so that it is even moredifficult to perform cutting in the free dross state; and to make clearthe state of dross adhesion, etc., in the energy density I/m regions ofthe arc jet at which cutting is not conventionally performed. FIG. 3shows reference characters designating dimensions, etc., in the plasmatorch 60. The same component parts are given the same referencecharacters, and will not be described below.

(1) Principal dimensions in the plasma torch 60 used in the experiment:

Outer diameter db_(x) of electrode 62=5.5 mm

Diameter da_(x) of lower end of electrode 62=2.7 mm

Taper angle α_(x) of electrode 62=90°

Inner diameter Da_(x) of swirler member 63=8.5 mm

Length corresponding to parallel section length Hd of plasma torch 1=0mm

Diameter Dd_(x) of velocity reduction space 61a=2.0 mm

Height Ha_(x) of velocity reduction space 61a=1.5 mm

Nozzle 61 angle θ_(x) nozzle 61 below velocity reduction space 61a=120°

Nozzle 61 angle φ_(x) =90°

Nozzle 61 orifice diameter De=0.8 mm

Distance Hb_(x) between lower end of electrode 62 and velocity reductionspace 61a=1.3 mm

Length Hc_(x) of nozzle restriction section 61a=2.6 mm

(2) Cutting conditions:

Arc current value I=37 A

Type of operating gas=oxygen

Operating gas flow rate m (following four values)

=11.5×10⁻⁵ kg/S (Line L1 of FIG. 4)

=9.5×10⁻⁵ kg/S (Line L2 of FIG. 4)

=7.5×10⁻⁵ kg/S (Line L3 of FIG. 4)

=6.0×10⁻⁵ kg/S (Line L4 of FIG. 4)

Stand-off=2 mm

Workpiece=Soft steel plate

Plate thickness=6 mm

(3) Experimental results:

The results of this experiment are shown in FIG. 4. In this experimentdross adhesion was observed in the L1 and L2 regions, that is theregions having a small energy density I/m, where a large amount of aconventional operating gas was used. It was found that in the line L4(energy density I/m=6.2×10⁵ (A·S/kg)] and the line L3 [energy densityI/m=4.9×10⁵ (A·S/kg)] regions where a small amount of operating gas wasused, that is, where energy density I/m was large, it is possible toperform cutting in a dross free state. However, although only smallamounts of dross adhesion occurred at a cutting velocity of 60˜100cm/min, this depends on the plate thickness, current value, etc. Theinventors have found out from many experimental results that when theenergy density I/m is larger than approximately 4×10⁵ (A·S/kg), it ispossible to achieve cutting in a free dross state. However, theinventors have also found out that when cutting is performedsuccessively for a large number of times, double arc occurs and that, aswill be described below, durability of the plasma arc is decreased.

EXPERIMENTAL EXAMPLE 2: Number of cumulative occurrences of double arcs

The double arc occurrence conditions and dross adhesion were checkedusing the plasma torch 1 of FIG. 1b, which is a plasma torch of thepresent invention. Cutting (described later) was performed with threenozzles 9 having the same shape. The conventional plasma torch 60 havingthe same dimensions as those of the plasma torch used in theaforementioned first experimental example was used, except that thenozzle orifice diameter De was 0.6 mm.

(1) Principal dimensions in the plasma torch 1 used in the experiment:

Diameter da of lower end 3b of electrode=2.7 mm

Outer diameter db of electrode 3=5.5 mm

Taper angle α=40°

Inner diameter Dc of nozzle 9=8.5 mm

Length Hd of entrance section L=2.7 mm

Diameter Dd of velocity reduction space N=4 mm

Height Ha of velocity reduction space N=0.6 mm

Angle θ of acceleration space P=120°

Angle φ of acceleration section m=60°

Nozzle orifice diameter De=0.6 mm

Length Hc of nozzle restriction section 9a=2.0 mm

(2) Cutting conditions (same for both plasma torch 1 and plasma torch60):

Arc current value I=27 A

Energy density I/m=6.5×10⁵ A·S/kg

Stand-off=2 mm

Type of operating gas=oxygen

Workpiece=Soft steel plate

Plate thickness=1.6 mm

(3) Experimental results:

Piercing was started to perform a 10-cm straight cut and this wasrepeated for 1000 times, and the number of cumulative occurrences ofdouble arcs were examined. The double arc occurrences were measured fromchanges in the input voltage values, while dross adhesion was visuallymeasured. FIG. 5 shows the relationship between the number of piercingsand the number of cumulative occurrences of double arcs.

Experimental results showed that when the conventional plasma torch 60was initially used, dross adhesion did not occur. However, when thenumber of cutting operations approached 600 times, double arcscumulatively occurred 50 times, so that slight dross adhesion wasobserved. When the number of cutting operations exceeded 800 times, theoccurrences of double arcs increased rapidly, so that a large amount ofdross adhesion was observed. From the many experimental results, thepresent inventors confirmed that when the energy density I/m is greaterthan approximately 4×10⁵ A·S/kg, cutting in a dross free state isachieved. However, the inventors also found that when the cutting isrepeated for a large number of times, double arcs as well as largeamounts of dross adhesion were observed, with reduced durability of theplasma torch.

The experimental results showed that when the plasma torch 1 of thepresent invention was used, double arcs occurred cumulatively only about50 times when the cutting operations were repeated for 1000 times, asshown by lines L8, L9, and L10. In this case, no dross adhesion wasobserved on the cut section. Compared to the conventionally-constructedplasma torch, even when the same energy density I/m is applied, theplasma torch of the invention has more power to stably maintain the arccolumn with respect to the electrode axis, so that even when theoperating gas flow rate is small at approximately 4.2×10⁻⁵ kg/S, thereis less instability of the arc column, and cutting can be stablyperformed for a long period of time without dross adhesion, that is in adross free state.

EXPERIMENTAL EXAMPLE 3: Dross adhesion height with various nozzlediameters

FIG. 6 illustrates the experimental results. FIG. 6 is a graph showingthe relationship between gas flow rate and current allowing cuttingwhere no dross adhesion height is visually measured or allowing cuttingin a dross free state, when changes are made in the cutting currentusing various nozzle orifice diameters De in the plasma torch of thepresent invention. The figure shows that, for example, when the arccurrent value I is 40 A, the operating gas flow rate m limit allowingcutting in a dross free state is approximately 10×10⁻⁵ kg/s (representedby O in the figure), while in regions where the flow rate is less thanthis value, it is possible to perform cutting in a dross free state.

From this experiment, the limit value of energy density I/m=4×10⁵A·S/kg. This means that the dross free region is located where theenergy density I/m is greater than this limit value.

EXPERIMENTAL EXAMPLE 4: Cutting velocity measurement

In the experiment, the plasma torch 1 of the present invention and theconventional plasma torch 60 were used to examine the cutting velocitiesallowing cutting in a dross free state. The main conditions were aworkpiece plate thickness of 1.6 mm, a nozzle orifice diameter De of 0.6mm, an arc current value I of 27 A, oxygen as operating gas, and anoperating gas flow rate at which the energy density I/m is greater than4×10⁵ A·S/kg. Cutting at various velocities revealed that the dross freeregion of the plasma torch 1 was approximately 100˜190 cm/min, while thedross free region of the plasma torch 60 was approximately 100˜155cm/min. This means that at the region where I/m≧4×10⁵ A·S/kg, it ispossible to perform cutting in a dross free state, while, at the sametime, the cutting velocity is a practical velocity, with the plasmatorch 1 of the present invention being about 1.23 times faster than theconventional ones.

EXPERIMENTAL EXAMPLE 5: Measurement by enlarged plasma torch model

This experiment was conducted to find out preferable dimensions andshapes for the plasma torch 1 of the present invention. Accordingly, tofind out the relationship of plasma torch shape and the swirling currentstrength and uniformity, plasma torches of a model having five times thedimensions of the plasma torch 1 were manufactured for various standardsto measure the static pressure at each of the points in the torchinterior where operating gas flows. The reference characters, etc., ofthe present plasma torch is the same as those of the plasma torch 1, sothat they will not be described here.

(1) Common dimensional forms of plasma torches and gas flow rate:

Nozzle orifice diameter De=3.0 mm

Length Hc of nozzle orifice=3De

Operating gas (oxygen) flow rate 9.5×10⁻⁴ kg/S (2)

(2) Measurement position of static pressure in plasma torch interior:

Center of lower end 3b of electrode (static pressure at this positioncalled Pe)

Wall face of lower portion of velocity reduction space N (staticpressure at this position called Pvr)

(3) Experimental results:

The experimental results were as follows:

a) FIG. 9 shows the relationship between the (parallel section length Hdof entrance section L/nozzle diameter De) and the static pressure Pe,where the height Ha of the velocity reduction space N=nozzle orificediameter De, the distance Hb between the lower end 3b of the electrodeand the velocity reduction space N is 0, and the diameter Dd of thevelocity reduction space N=7 De. Since centrifugal force acts upon theoperating gas, which is a fluid, swirling currents with a largerswirling velocity component V.sub.θ (see FIG. 2) causes a lower staticpressure Pe at the lower end 3b of the electrode 3. From the manyexperimental results described above, it is preferable that the staticpressure Pe be not more than about 0.7 kg/cm², so that the preferablerange of the parallel section length Hd of entrance section L/nozzleorifice diameter De is 0≦Hd/De≦7.

b) The relationship between the angle φ of acceleration section M andthe static pressure Pe, when, for example, Ha=De, Hb=0, and Dd=7 De asin the aforementioned a). The results showed that the angle φ at whichthe static pressure Pe equals the same desirable value as in theaforementioned a) of not more than about 0.7 kg/cm² falls in the rangeof 30°≦φ≦100°.

c) A desirable angle θ acceleration space P was selected to maintain thestability of the arc jet. More specifically, when θ<90°, the length fromthe bottom face of the velocity reduction space N to the nozzlerestriction section 9a becomes too long, so that the arc jet becomesmore unstable. On the other hand, when θ>150°, the operating gas israpidly accelerated to the nozzle restriction section 9a, so that theflow often becomes unstable. Therefore the angle θ is preferably in therange of 90°≦θ≦150°.

d) FIG. 10 shows the relationship between the (height Ha of velocityreduction space N/nozzle orifice diameter De) to the static pressure Pvrof the wall at the lower portion of the velocity reduction space N. Thegraph shows the result when the distance Hb=0 and the diameter Dd=7 De.A higher static pressure Pvr value forms a more effective pressuredistribution at the lower face of the velocity reduction space N. Thestatic pressure Pvr is preferably greater than about 1.2 kg/cm² for itto exist in an extremely stable state. Therefore, although anappropriate Ha/De value would be Ha/De≦2.5, since when Ha/De<0.5 aproper discharge gap cannot be obtained, it is preferably in the rangeof 0.5≦Ha/De≦2.5.

e) Examination of the relationship between the (diameter Dd/nozzleorifice diameter De) and the static pressure Pe showed that a desirablestatic pressure Pe value can be obtained, that is, the center of the arcjet in the plasma torch enters an effective low pressure space whenDd/De lies within the preferable range of 4≦Dd/De≦10.

f) Experiments were carried out, under the condition that the heightHa=the nozzle diameter De and the diameter Dd=7 De, to obtain apreferable distance Hb between the lower end 3b of the electrode 3 andthe velocity reduction space N. Examination of the relationship betweenthe (distance Hb/nozzle diameter De) and the static pressure Pe revealedthat the preferable static pressure is obtained when it lies within thepreferable range of -0.4≦Hb/De≦0.6.

EXPERIMENTAL EXAMPLE 6: Measurement by plasma torch 1

The experiment was conducted to obtain preferable dimensions as regardsthe length Hc of the nozzle orifice of the plasma torch 1 of the presentinvention. FIG. 11 shows the relationship between (length Hc of nozzlediameter De/nozzle orifice diameter De) and the double arc occurrencelimiting current Ic. In this case, the nozzle diameter De=0.6 mm and theoperating gas used was oxygen. From various experiments, it can bethought that (length Hc/nozzle diameter De) value of not more than 4 isappropriate to obtain the required double arc occurrence limitingcurrent Ic of, for example, about 30 A or more. However, when Hc/De<2.5,the arc jet cannot be sufficiently contracted by the thermal pincheffect, which means that good cutting quality cannot be obtained.Therefore, the preferable range is 2.5≦Hc/De≦4.

With the constructions in Examples 5 and 6, the plasma torch 1 allowscutting in a dross free state, and, at the same time, it can be designedbased on a wide range of dimensional forms, when necessary.

INDUSTRIAL APPLICABILITY

The present invention is effective in that it provides a plasma torchcapable of cutting in a dross free state, made possible by increasedenergy density of the arc jet, and of an operation efficiency which isnot reduced even with a low operating gas flow rate since it can stablymaintain the arc jet in the plasma torch, and which has high double arcresistance and high durability.

What is claimed is:
 1. A plasma torch comprising:an electrode having alongitudinal axis, an upper portion, an intermediate portion, a lowerportion, and a lower end face, said lower end face having a diameter da;an annular nozzle body having an upper portion, an intermediate portion,and a lower portion, said nozzle body being positioned coaxially withand about said electrode so as to form an annular entrance sectionbetween said intermediate portion of said nozzle body and saidintermediate portion of said electrode and to form an annular taperedsection between said intermediate portion of said nozzle body and saidlower portion of said electrode; an annular swirler member positionedcoaxially with said electrode between said upper portion of saidelectrode and said upper portion of said nozzle body to form an annulargas passage between said swirler member and said electrode; an annularinsulating member positioned coaxially with said electrode between saidupper portion of said electrode and said swirler member; said swirlermember having a plurality of ejection holes formed therein in a planesubstantially perpendicular to said longitudinal axis, said ejectionholes extending approximately tangential to said annular gas passage togenerate jets therein with a swirling velocity component; wherein saidlower portion of said nozzle body has a nozzle orifice formed thereinopening to an exterior of said nozzle body, said nozzle orifice having adiameter De and an axial length Hc; wherein said lower portion of saidnozzle body has a velocity reduction space formed therein between saidelectrode and said orifice and below said annular tapered section;wherein said velocity reduction space is in the form of a cylindricallyshaped space which is coaxial with said longitudinal axis and which hasa diameter Dd and an axial height Ha; wherein said diameter Dd of saidvelocity reduction space is greater than said diameter da of said lowerend face of said electrode; and wherein said diameter Dd of saidvelocity reduction space is greater than said axial height Ha of saidvelocity reduction space.
 2. A plasma torch in accordance with claim 1,wherein a ratio of Dd/Ha is at least 4/0.6.
 3. A plasma torch inaccordance with claim 1, wherein a ratio of Dd/da is at least 4/2.7. 4.A plasma torch in accordance with claim 3, wherein a ratio of Dd/Ha isat least 4/0.6.
 5. A plasma torch in accordance with claim 1, whereinsaid axial height Ha of said velocity reduction space is in the range of0.5De to 2.5De.
 6. A plasma torch in accordance with claim 1, whereinsaid diameter Dd of said velocity reduction space is in the range of 4Deto 10De.
 7. A plasma torch in accordance with claim 1, wherein an axialdistance Hb between said lower end face of said electrode and an upperend of said velocity reduction space is in the range of --0.4De to0.6De.
 8. A plasma torch in accordance with claim 1, wherein said axiallength Hc of said nozzle orifice is in the range of 2.5De to 4De.
 9. Aplasma torch in accordance with claim 1, wherein an axial length Hd ofsaid entrance section is in the range of 0 to 7De.
 10. A plasma torch inaccordance with claim 1, wherein said intermediate portion of saidnozzle body which forms said annular tapered section has a taper angle φwhich is in the range of 30° to 100°.
 11. A plasma torch in accordancewith claim 1, wherein said nozzle body has a conical accelerationsection converging downwardly and inwardly from said velocity reductionspace to said nozzle orifice, and wherein said conical accelerationsection has a taper angle θ which is in the range of 90° to 150°.
 12. Aplasma torch in accordance with claim 11, wherein said intermediateportion of said nozzle body which forms said annular tapered section hasa taper angle φ which is in the range of 30° to 100°.
 13. A plasma torchin accordance with claim 1, wherein said axial height Ha of saidvelocity reduction space is in the range of 0.5De to 2.5De;wherein saiddiameter Dd of said velocity reduction space is in the range of 4De to10De; wherein an axial distance Hb between said lower end face of saidelectrode and an upper end of said velocity reduction space is in therange of -0.4De to 0.6De; wherein said axial length Hc of said nozzleorifice is in the range of 2.5De to 4De; wherein an axial length Hd ofsaid entrance section is in the range of 0 to 7De; wherein saidintermediate portion of said nozzle body which forms said annulartapered section has a taper angle φ which is in the range of 30° to100°; wherein said nozzle body has a conical acceleration sectionconverging downwardly and inwardly from said velocity reduction space tosaid nozzle orifice; and wherein said conical acceleration section has ataper angle θ which is in the range of 90° to 150°.
 14. A plasma torchin accordance with claim 13, wherein a ratio of Dd/Ha is at least 4/0.6.15. A plasma torch in accordance with claim 13, wherein a ratio of Dd/dais at least 4/2.7.
 16. A plasma torch in accordance with claim 15,wherein a ratio of Dd/Ha is at least 4/0.6.
 17. A plasma torch inaccordance with claim 15, wherein said plasma torch provides an arc jetenergy density greater than 4×10⁵.
 18. A plasma torch in accordance withclaim 1, wherein said plasma torch provides an arc jet energy densitygreater than 4×10⁵.